Mastering the Art of Polyurethane Vacuum Casting (script)
Hello and welcome. This is Gordon Styles, the president and founder of Star Rapid. I’ve been involved with rapid prototyping, tool making, machining and new product introduction for more than 35 years, and I’m happy to be back with you once again for another installment of Serious Engineering for serious engineers.
As you know, I’ve spent millions of years floating through the depths of intergalactic space, so I’m pretty comfortable being around and working with vacuum. And vacuum, despite being the definition of “nothing”, also happens to be useful for all kinds of “somethings”. For example, cleaning crumbs from sofa cushions; helping secret agents cling to skyscrapers to foil international terrorism; supercharging the last of the V8 Interceptors; and making really nice low-volume parts and prototypes.
“But Master Styles”, you say, “how can something come from nothing?” That is the age-old question my young apprentice, is it not?
Allow me to be your guide to making polyurethane vacuum cast parts. You’ll sometimes hear this referred to as “urethane” casting, which is not technically correct but for some reason you Yanks out there say it all the time. Ahem.
This is the process we have at Star, but there may be slightly different techniques out there. If you have some suggestions, we’d love to hear about them in the comments section below.
No matter the variations, vacuum casting is a great solution for rapid prototyping and new product development. If you haven’t seen the quality for yourself, you’d be amazed at the beautiful results you can achieve, from start to finish, in about a week’s time. And that’s without having to invest in hard tooling. So, if you need a few parts for prototype testing, display models, proof-of-concept or a crowdfunding campaign, and they need to be spot-on to the real thing, this is for you.
1. Master Patterns
Vacuum casting starts with a master pattern. Technically they can be any physical solid that’s a copy of the finished part. It needs to be rigid enough to hold its shape at temperatures of about 50~70 C at the most.
You can CNC machine master patterns if you want but we don’t normally recommend it. The reason is that CNC milling makes radiused corners using round cutters, so the master pattern ends up not being a facsimile of the CAD model.
Instead, we prefer 3D SLA printing. SLA is fast, inexpensive, accurate and it produces an excellent surface finish. This is important because vacuum casting will accurately reproduce all surface details, including the ones you don’t want. So, it really pays to take your time at this stage, carefully sanding and priming the master pattern to get it just right.
We will also add tape along the split line to help release the part later. And some holes or other deep features, like ribs or pockets, may need to be plugged because the silicone isn’t strong enough to hold small shapes when the master pattern is released.
We make casting boxes to hold the liquid silicone when it’s poured around the pattern. We join our plates with hot glue so they can be disassembled later. Boxes should only be a little bigger than the pattern itself to avoid wasting material and to make removal easier.
Now, while the casting boxes are being prepared, the silicone is being mixed with a hardener inside a vacuum chamber to remove air bubbles.
Master patterns are suspended in the casting box at a point roughly equidistant to all of the sides. We might also add additional rods to support larger or oddly shaped pieces. The silicone is poured in carefully from one corner, which helps to avoid introducing any air pockets.
We then put the boxes in a vacuum casting machine to degass them, and then they’re cured in an oven for 8 ~ 16 hours at about 40C, depending on the part size.
“But Master Styles”, you ask, “where are my copies?” Patience my young Padawan apprentice, you must learn patience.
4. Opening the mold
Now we need our Jedi skills. The mold needs to be opened along the split line with a series of incisions. We must get right to the edge of the pattern without damaging it, and to do this it helps to use a strong directional light source, so the edge is clearly illuminated.
Notice that we make jagged, saw-tooth cuts on this seam. This helps us reseal the mold in the correct orientation later.
Once the mold is opened – which can take a bit of muscle – we remove the master pattern and expose the empty cavity within, a perfect negative image of the original. Ta Dah!
5. Making Copies
To make copies you need pourable polyurethane casting resin. There are many formulations available, and they basically mimic the mechanical properties of standard PIM resins like nylon or ABS, or soft TPE elastomer rubber. These resins can also be pigmented in advance, so the color is molded in.
We like to warm our resins well in advance and make sure that they’re completely mixed. If there’s any resin left over after a pour – and there often is – we make sure the bottle is sealed with argon gas before putting it back into storage. This keeps it from absorbing moisture which is a foaming agent for polyurethane.
Next, we add a feed tube to the mold as well as a few air channels leading away from the cavity. The mold is then resealed, stapled and taped shut. We put the mold into the vacuum casting machine, where an automatic arm pours the resin into the cavity. Here again, vacuum is used to remove trapped air bubbles and fully pack the cavity. And because of the vacuum, air molecules become about 50X smaller, so they don’t have a great affect on the features of the cast part.
After curing we open the mold and Voila! A perfect copy of the original pattern. Silicone molds can be used for up to about 20 castings before they need to be replaced. This is because they chemically break down when exposed to the resin.
So, there you have it, a primer on how to use polyurethane vacuum casting for rapid prototypes and low volume finished parts. If we missed something or you’d like to know more, please drop us a comment below. Don’t forget to ring the bell, like us and subscribe. And of course, always use the force for good and never for evil.